1. Field of the Invention
The present invention relates to achieving a level surface on an exposed surface of a viscous fluid. More particularly, the present invention relates to maintaining a level surface on a pool of adhesive material for applying the adhesive material to the lead fingers by contacting the lead fingers with the pool of adhesive material.
2. State of the Art
Higher performance, lower cost, increased miniaturization of semiconductor components, and greater packaging density of integrated circuits are goals of the computer industry. One way to reduce the overall cost of a semiconductor component is to reduce the manufacturing cost of that component. Lower manufacturing costs can be achieved through faster production and/or reduction in the amount of materials used in fabricating the semiconductor component.
One area where faster production and reduction in material usage can be achieved is in the area of lead frame attachment to semiconductor dice. U.S. Pat. No. 5,286,679 issued Feb. 15, 1994 to Famworth et al. (xe2x80x9cthe ""679 patentxe2x80x9d), assigned to the assignee of the present invention and hereby incorporated herein by reference, teaches attaching leads to a semiconductor device with adhesive material in a xe2x80x9clead-over-chipxe2x80x9d (xe2x80x9cLOCxe2x80x9d) configuration. The ""679 patent teaches applying a patterned thermoplastic or thermoset adhesive layer to a semiconductor wafer. The adhesive layer is patterned to keep the xe2x80x9cstreetsxe2x80x9d on the semiconductor wafer clear of adhesive for saw cutting and to keep the wire bonding pads on the individual dice clear of adhesive for wire bonding. Patterning of the adhesive layer is generally accomplished by hot or cold screen/stencil printing or dispensing by roll-on. Following the printing and baking of the adhesive layer on the semiconductor wafer, the individual dice are singulated from the semiconductor wafer. During packaging, each adhesive coated die is attached to lead fingers of a lead frame by heating the adhesive layer and pressing the lead fingers onto the adhesive. If the adhesive layer is formed of a thermoset material, a separate oven cure is required. Furthermore, the adhesive layer may be formulated to function as an additional passivating/insulating layer or alpha barrier for protecting the packaged die.
Although the teaching of the ""679 patent is an effective method for attaching leads in a LOC configuration, it is difficult to achieve an adequate profile on the adhesive such that there is sufficient area on the top of the adhesive to attach the lead fingers. The process disclosed in the ""679 patent is illustrated in FIGS. 23-29. FIG. 23 illustrates a side, cross-sectional view of a semiconductor substrate 302 with a bond pad 304, wherein a stencil or a screen print template 306 has been placed over the semiconductor substrate 302, generally a silicon wafer. The stencil or screen print template 306 is patterned to clear the area around the bond pads 304 and to clear street areas 308 for saw cutting (i.e., for singulating the substrate into individual dice). An adhesive material 310 is applied to the stencil or screen print template 306, as shown in FIG. 24. Ideally, when the stencil or screen print template 306 is removed, adhesive prints 312 are formed with vertical sidewalls 314 and a planar upper surface 316, as shown in FIG. 25. However, since the adhesive material 310 must have sufficiently low viscosity to flow and fill the stencil or screen print template 306, as well as allow for the removal of the stencil or screen print template 306 without the adhesive material 310 sticking thereto, the adhesive material 310 of the adhesive prints 312 will spread, sag, or flow laterally under the force of gravity after the removal of the stencil or screen print template 306, as shown in FIG. 26. This post-application flow of adhesive material 310 can potentially cover all or a portion of the bond pads 304 or interfere with the singulating of the semiconductor wafer by flowing into the street areas 308.
Furthermore, and of even greater potential consequence than bond pad or street interference is the effect that the lateral flow or spread of adhesive material 310 has on the adhesive material upper surface 316. As shown in FIG. 27, the adhesive material upper surface 316 is the contact area for lead fingers 318 of a lead frame 320. The gravity-induced flow of the adhesive material 310 causes the once relatively well-defined edges 322 of the adhesive material to curve, resulting in a loss of surface area 324 (ideal shape shown in shadow) for the lead fingers 318 to attach to. This loss of surface area 324 is particularly problematical for the adhesive material upper surface 316 at the longitudinal ends 326 thereof. At the adhesive material longitudinal ends 326, the adhesive material flows in three directions (to both sides as well as longitudinally), causing a severe curvature 328, as shown in FIGS. 28 and 29. The longitudinal ends of the adhesive print on patch flow in a 180xc2x0 flow front, resulting in blurring of the print boundaries into a curved perimeter. This curvature 328 results in complete or near complete loss of effective surface area on the adhesive material upper surface 316 for adhering the outermost lead finger closest to the adhesive material longitudinal end 326 (lead finger 330). This results in what is known as a xe2x80x9cdangling lead.xe2x80x9d Since the lead finger 330 is not adequately attached to the adhesive material longitudinal end 326, the lead finger 330 will move or bounce when a wirebonding apparatus (not shown) attempts to attach a bond wire (not shown) between the lead finger 330 and its respective bond pad 304 (shown from the side in FIG. 29). This movement can cause inadequate bonding or non-bonding between the bond wire and the lead finger 330, resulting in the failure of the component due to a defective electrical connection.
LOC attachment can also be achieved by attaching adhesive tape, preferably insulative, to an active surface of a semiconductor die, then attaching lead fingers to the insulative tape. As shown in FIG. 30, two strips of adhesive tape 410 and 410xe2x80x2 are attached to an active surface 412 of a semiconductor die 404. The two adhesive tape strips 410, 410xe2x80x2 run parallel to and on opposing sides of a row of bond pads 406. Lead fingers 402, 402xe2x80x2 are then attached to the two adhesive tape strips 410, 410xe2x80x2, respectively. The lead fingers 402, 402xe2x80x2 are then electrically attached to the bond pads 406 with bond wires 408. Although this method is effective in attaching the lead fingers 402, 402xe2x80x2 to the semiconductor die 404, this method is less cost effective than using adhesive since the cost of adhesive tape is higher than the cost of adhesive material. The higher cost of the adhesive tape is a result of the manufacturing and placement step which are required with adhesive tapes. The individual tape segments are generally cut from a larger tape sheet. This cutting requires precision punches with extremely sharp and accurate edges. These precision punches are expensive and they wear out over time. Furthermore, there is always waste between the segments which are punched out, resulting in high scrap cost. Moreover, once punch out is complete, the tape segments are placed on a carrier film for transport to the die-attach site. Thus, there are problems with placement, alignment, and attachment with film carriers, plus the cost of the film carrier itself.
LOC attachment can further be achieved by placing adhesive material on the lead fingers of the lead frame rather than on the semiconductor substrate. As shown in FIG. 31, the adhesive material 502 may be spray applied on an attachment surface 504 of lead fingers 506. However, the viscous nature of the adhesive material 502 results in the adhesive material 502 flowing down the sides 508 of the lead finger 506 and collecting on the reverse, bond wire surface 510 of the lead finger 506, as shown in FIG. 32. The adhesive material 502 which collects and cures on the bond wire surface 510 interferes with subsequent wirebonding, which, in turn, can result in a failure of the semiconductor component. The flow of adhesive material 502 for the attachment surface 504 to the bond wire surface 510 can be exacerbated if the lead fingers 506 are formed by a stamping process rather than by etching, the other widely employed alternative. The stamping process leaves a slight curvature 512 to edges 514 of at least one surface of the lead finger 506, as shown in FIG. 33. If an edge curvature 512 is proximate the lead finger attachment surface 504, the edge curvature 512 results in less resistance (i.e., less surface tension) to the flow of the adhesive material 502. This, of course, results in the potential for a greater amount of adhesive material 502 to flow to the bond wire surface 510.
Furthermore, present methods of adhesive material application on a surface (whether of the semiconductor die or the lead fingers) tend to waste adhesive material. For example, spray application loses a great deal of adhesive material because not all of the sprayed adhesive material attaches to the target surface. As another example, the patterning of an adhesive layer on a semiconductor die, such as described in the ""679 patent, results in a substantial area of the adhesive pattern not being utilized to attach leads.
Thus, is can be appreciated that it would be advantageous to develop a method and apparatus for rapidly applying an adhesive material to a lead finger with little waste of adhesive material.
The present invention relates to a method for applying an adhesive material to lead fingers of a lead frame wherein surfaces of the lead fingers which receive the adhesive material face downward to contact a pool of adhesive material. Preferably, the adhesive material cures with the lead frame in this downward facing position. The advantages of placing viscous material, such as an adhesive material, in a downward facing position is described in U.S. patent application Ser. No. 08/709,182 by Tongbi Jiang and Syed S. Ahmad filed Sep. 6, 1996, assigned to the assignee of the present invention and hereby incorporated herein by reference. An adhesive reservoir retaining the adhesive material can be shaped such that the exposed surface (pool) of the adhesive material is in a precise location. When the lead fingers contact the exposed surface of the adhesive material, the adhesive material attaches to only specific, desired portions of the lead fingers.
Rather than gravitational forces causing the adhesive material to flow and expand as when on top of the lead frame, the gravitational forces on the inverted lead frame maintain the shape and boundary definition of the adhesive material. It is, of course, understood that the viscous adhesive material must be compatible with the lead finger material so as to adhere thereto and must not be of such a low viscosity that it drips when the lead fingers are removed from contact with the adhesive material pool. Preferably, the viscous materials have viscosities between about 1000 cps and 500,000 cps.
Of critical importance to the application of the adhesive material to the lead fingers in the method described above is the levelness of the exposed surface of the adhesive material of the pool. If the exposed surface is not level, the lead fingers may extend too deeply into the adhesive material. When this occurs, the adhesive material may wet sides of the lead finger and may even wet a bond wire surface of the lead finger. If the adhesive material wets the bond wire surface, the adhesive material may interfere with a wirebonding step subsequent to LOC attachment of the lead fingers to an active surface of a semiconductor die.
A preferred method of controlling the levelness of the exposed surface is by attaching a coating stencil having small apertures, such as a screen or a plate with slots, to the adhesive reservoir, such that the only outlet for the adhesive material is through the apertures in the coating stencil. The adhesive material is thus forced through the coating stencil. The surface tension between walls of the small apertures and the adhesive material flattens out the exposed surface of the adhesive material. This allows a larger area to be printed with a more uniform thickness layer than if the coating stencil is not used. It is, of course, understood that the flatness or shape of the adhesive material can be controlled by the design of the apertures of the coating stencil. Thus, the present invention is an efficient way to use the surface tension of the adhesive material to control surface area and thickness of the adhesive material available for application to lead fingers.